GE GFK-1541B manual Sequencing Communications Requests, Monitoring the Channel Open Bit

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Monitoring the Communications Channel

Once you have a working ladder program, you can use the status data to monitor your communications activity and take the desired action upon certain events.

Monitoring the COMMREQ Status Word

It is critical to monitor the CRS word for each COMMREQ function you initiate. First, zero the associated CRS word before executing the COMMREQ function. Then when the CRS word becomes non-zero, you know the Ethernet Interface has updated it. If the CRS word is updated to a one (1), then the Command Block was processed successfully by the Ethernet Interface. If the CRS word is updated to a value other than 1, then an error has occurred in processing the Command Block.

Do not use data received from a server until the CRS word for that channel is 1 or the Data

Transfer bit goes to 1.

Monitoring the Channel Open Bit

This bit is 1 when a Channel has successfully established a connection with a remote server, and is 0 when a Channel has been closed.. The Channel Open Bit is meaningful whenever the PLC CPU is in run mode and the particular channel is being used by Modbus/TCP. The Channel Open Bit is set at the same time the successful status is returned to the CRS word for the Open Modbus/TCP Connection COMMREQ.

Sequencing Communications Requests

If the Ethernet Interface receives Command Blocks from the PLC CPU faster than the Interface can process them, the Interface will log an exception event 08, Entry 2=0024H and will log the PLC Fault Table entry:

“Backplane Communications with PLC Fault; Lost Request”

Only one COMMREQ function per channel can be pending at one time. A COMMREQ function is pending from the time it is initiated in the ladder program until its CRS word has been updated to a non-zero value by the Ethernet Interface.

4-34 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B